1. Field
Example embodiments relate generally to an image sensor, and more particularly to a depth pixel that captures depth data of an object by measuring a time of flight (TOF) of a modulation light and an image pick-up apparatus that includes the depth pixel.
2. Description of the Related Art
Recently, image sensors configured to acquire depth data using a photo gate have been suggested. The image sensor acquires the depth data of a subject by calculating the TOF of an illumination light or a modulation light based on the difference between an amount of photo-electrically converted charges stored in a first photo cell that operates in a timing of the illumination light and a second photo cell that operates in a 180° phase-delayed timing. Thus, in order to improve the accuracy of the depth image in depth capturing technology that uses a photo gate or a photodiode, it is necessary to increase a sensitivity of the photo gate or the photodiode or to increase an intensity of a reflection light by increasing a power of the illumination light. However, increasing the power of the illumination light may cause a great amount of power to be consumed. In addition, if a modulation frequency of the illumination light is increased or a duty cycle of a modulation waveform is reduced to improve accuracy, the maximum distance or depth that may be sensed may be reduced. At this time, the maximum sensing distance Zmax may be determined based on the waveform and the duty cycle of the illumination light.
The accuracy of the depth image is proportional to the intensity of the illumination light and inversely proportional to the duty cycle of the illumination cycle. Thus, the accuracy of the depth image may be improved by increasing the intensity of the illumination light and reducing the duty cycle of the illumination light. However, if a noise component of an ambient light, such as a solar light, is significantly greater than a magnitude of a scanning light in an image sensor, then it is necessary to compensate for the noise component. If the full well capacity of the photo cell is increased to prevent saturation of the photo cell caused by the ambient light, the conversion gain/sensitivity is reduced so that the signal-to-noise ratio (SNR) is reduced, resulting in an increase of a depth error.